Question about robot design

Hi, i would like to know how to make the robot move in straight line , because last year , when i let the robot move forward , it bends to right , both motors were the same type but they were used motors ... are there any mechanism to let the robot move in a striaght line ??

thanks alot , very usefull forums .

i wish to win in egypt so i can meet ya guys in vietnam ,inshallah
Ahmed , EGYPT